From laolu32 at gmail.com Thu Mar 24 22:52:42 2022 From: laolu32 at gmail.com (Olaoluwa Osuntokun) Date: Thu, 24 Mar 2022 15:52:42 -0700 Subject: [Lightning-dev] Dynamic Commitments Part 2: Taprooty Edition Message-ID: Hi y'all, ## Dynamic Commitments Retrospective Two years-ish ago I made a mailing list post on some ideas re dynamic commitments [1], and how the concept can be used to allow us to upgrade channel types on the fly, and also remove pesky hard coded limits like the 483 HTLC in-flight limit that's present today. Back then my main target was upgrading all the existing channels over to the anchor output commitment variant, so the core internal routing network would be more resilient in a persistent high fee environment (which hasn't really happened over the past 2 years for various reasons tbh). Fast forward to today, and with taproot now active on mainnet, and some initial design work/sketches for taproot-native channels underway, I figure it would be good to bump this concept as it gives us a way to upgrade all 80k+ public channels to taproot without any on chain transactions. ## Updating Across Witness Versions w/ Adapter Commitments In my original mail, I incorrectly concluded that the dynamic commitments concept would only really work within the confines of a "static" multi-sig output, meaning that it couldn't be used to help channels upgrade to future segwit witness versions. Thankfully this reply [2] by ZmnSCPxj, outlined a way to achieve this in practice. At a high level he proposes an "adaptor commitment" (similar to the kickoff transaction in eltoo/duplex), which is basically an upgrade transaction that spends one witness version type, and produces an output with the next (upgraded) type. In the context of converting from segwit v0 to v1 (taproot), two peers would collaboratively create a new adapter commitment that spends the old v0 multi-sig output, and produces a _new_ v1 multi-sig output. The new commitment transaction would then be anchored using this new output. Here's a rough sequence diagram of the before and after state to better convey the concept: * Before: fundingOutputV0 -> commitmentTransaction * After fundingOutputV0 -> fundingOutputV1 (the adapter) -> commitmentTransaction It *is* still the case that _ultimately_ the two transactions to close the old segwit v0 funding output, and re-open the channel with a new segwit v1 funding output are unavoidable. However this adapter commitment lets peers _defer_ these two transactions until closing time. When force closing two transactions need to be confirmed before the commitment outputs can be resolved. However, for co-op close, you can just spend the v0 output, and deliver to the relevant P2TR outputs. The adapter commitment can leverage sighash anyonecanpay to let both parties (assuming it's symmetric) attach additional inputs for fees (to avoid introducing the old update_fee related static fee issues), or alternatively inherit the anchor output pattern at this level. ## Existing Dynamic Commitments Proposals Assuming this concept holds up, then we need an actual concrete protocol to allow for dynamic commitment updates. Last year, Rusty made a spec PR outlining a way to upgrade the commitment type (leveraging the new commitment type feature bits) upon channel re-establish [3]. The proposal relies on another message that both sides send (`stfu`) to clear the commitment (similar to the shutdown semantics) before the switch over happens. However as this is tied to the channel re-establish flow, it doesn't allow both sides to do things like only allow your peer to attach N HTLCs to start with, slowing increasing their allotted slots and possibly reducing them (TCP AIMD style). ## A Two-Phase Dynamic Commitment Update Protocol IMO if we're adding in a way to do commitment/channel upgrades, then it may be worthwhile to go with a more generalized, but slightly more involved route instead. In the remainder of this mail, I'll describe an alternative approach that would allow upgrading nearly all channel/commitment related values (dust limit, max in flight, etc), which is inspired by the way the Raft consensus protocol handles configuration/member changes. For those that aren't aware, Raft is a consensus protocol analogous to Paxos (but isn't byzantine fault tolerant out of the box) that was designed as a more understandable alternative to Paxos for a pedagogical environment. Typically the algorithm is run in the context of a fixed cluster with N machines, but supports adding/removing machines from the cluster with a configuration update protocol. At a high level the way this works is that a new config is sent to the leader, with the leader synchronizing the config change with the other members of the cluster. Once a majority threshold is reached, the leader then commits the config change with the acknowledged parties using the new config (basically a two phase commit). I'm skipping over some edge cases here that can arise if the new nodes participate consensus too early, which can cause a split majority leading to two leaders being elected. Applying this to the LN context is a bit simpler than a generalized protocol, as we typically just have two parties involved. The initiator is already naturally a "leader" in our context, as they're the only ones that can do things like trigger fee updates. ### Message Structure At a high level I propose we introduce two new messages, with the fields looking something like this for `commitment_update_propose`: * type: 0 (`channel_id`) * value: [`32*byte`:`chan_id`] * type: 1 (`propose_sig`) * value: [`64*byte`:`sig`] * type: 2 (`update_payload`) * value: [`*byte`:`tlv_payload`] and this `commitment_update_apply`: * type: 0 (`channel_id`) * value: [`32*byte`:`chan_id`] * type: 1 (`local_propose`) * value: [`*byte`:`commitment_update_propose`] * type: 2 (`remote_propose`) * value: [`*byte`:`commitment_update_propose`] ### Protocol Flow The core idea here is that either party can propose a commitment/channel param update, but only the initiator can actually apply it. The `commitment_update_propose` encodes the new set of updates, with a signature covering the TLV blob for the new params (more on why that's needed later). The `commitment_update_apply` includes up to _two_ `commitment_update_propose` messages (one for the initiator and one for the responder, as nested TLV messages). The `commitment_update_propose` message would be treated like any other `update_*` message, in that it takes a new commitment signature to properly commit/apply it. The normal flow takes the form of both sides sending a `commitment_update_propose` message, with the initiator finally committing both by sending a `commitment_update_apply` message. In the event that only the responder wants to apply a param change/update, then the initiator can reply immediately with a `commitment_update_apply` message that doesn't include a param change for their commitment (or they just echo the parameters if they're acceptable). ### Handling Retransmissions The role of the signature it to prevent "spoofing" by one of the parties (authenticate the param change), and also it serves to convince a party that they actually sent a prior commitment propose update during the retransmission phase. As the `commitment_update_propose` message would be retransmitted like any other message, if the initiator attempts to commit the update but the connection dies, they'll retransmit it as normal along with their latest signature. ### Nested TLV Param Generality The messages as sketched out here just have an opaque nested TLV field which makes it extensible to add in other things like tweaking the total number of max HTLCs, the current dust values, min/max HTLCs, etc (all things that are currently hard coded for the lifetime of the channel). An initial target would likely just be a `chan_type` field, with future feature bits governing _what_ type of commitment updates both parties understand in the future. In the past, when ideas like this were brought up, some were concerned that it wouldn't really be possible to do this type of updates while existing HTLCs were in flight (hence some of the ideas to clear out the commitment beforehand). I don't see a reason why this fundamentally _shouldn't_ be allowed, as from the point of view of the channel update state machine, all updates (adds/removes) get applied as normal, but with this _new_ commitment type/params. The main edge case we'll need to consider is cases where the new params make older HTLCs invalid for some reason. ## Conclusion Using the adapter commitment idea combined with a protocol for updating commitments on the fly, would potentially allow us to update all 80k+ segwit v0 channels to the base level of taprooty channels without any on chain transactions. The two transactions (open+close) must happen eventually, but by holding another layer of spends off-chain we can defer them (potentially indefinitely, as we have channels today that have been opened for over a year). Deploying a generalised on-the-fly dynamic commitment update protocol gives us a tool to future proof the _existing_ anchored multi-sig outputs in the chain, and also a way to remove many of the hard coded parameters we have today in the protocol. One overly inflexible parameter we have today in the network is the 483 HTLC limit. Allowing this value to float would allow peers to apply similar congestion avoidance algorithm that are used in TCP today, and also give us a way to protect the network against future unforeseen widespread policy changes (like a raising of the dust limit). -- Laolu [1]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-July/002763.html [2]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-July/002770.html [3]: https://github.com/lightning/bolts/pull/868 -------------- next part -------------- An HTML attachment was scrubbed... URL: